Igniter for igniting a fuel-air-mixture using HF corona discharge and engine fitted with such igniters

Abstract

An igniter for igniting a fuel-air-mixture in a combustion chamber, in particular in a combustion engine with one or several combustion chambers, by generating a high frequency corona discharge, said ignitiger comprising an assembly comprising an ignition electrode, an outer conductor surrounding the ignition electrode, which has a front end and a rear end, and an electrical insulator arranged between the ignition electrode and the outer conductor, wherein the ignition electrode is part of an electrical high-frequency resonant circuit, protrudes over the front end of the outer conductor, and is connected to an alternate current source or alternate voltage source which feeds the HF-resonant circuit, and delivers a high frequency alternate current or a high frequency alternate voltage, respectively. The ignition electrode is branched into more than four electrode branches extending away from the outer conductor, in different directions.

The invention claimed is: 
     
       1. An igniter for igniting a fuel-air-mixture in a combustion chamber, in particular in a combustion engine with one or several combustion chambers, by generating a high frequency corona discharge, said ignitiger comprising an assembly comprising an ignition electrode, an outer conductor surrounding the ignition electrode, which has a front end and a rear end, and an electrical insulator arranged between the ignition electrode and the outer conductor, wherein the ignition electrode is part of an electrical high-frequency resonant circuit, protrudes over the front end of the outer conductor, and is connected to an alternate current source or alternate voltage source which feeds the HF-resonant circuit and delivers a high frequency alternate current or a high frequency alternate voltage, respectively, wherein the ignition electrode is branched into more than four electrode branches extending away from the outer conductor, in different directions such that any two electrode branches are farthest apart from one another at their pointed ends and the solid angles between the directions to which any two neighboring electrode branches point with their pointed ends are so large that charge carrier clouds caused by HF-corona discharges and coming out of the electrode branches overlap at most in their edge regions. 
     
     
       2. An igniter according to  claim 1 , wherein the ends of the electrode branches are arranged in regular angular distances around the longitudinal axis of the center electrode. 
     
     
       3. An igniter according to  claim 1 , wherein the cross section of the electrode branches increases from their pointed ends towards the longitudinal median line of the ignition electrode. 
     
     
       4. An igniter according to  claim 1 , wherein the solid angle β between the directions, to which the ends of the electrode branches point, ranges between β=15° and β=60° and is preferably greater than 30°. 
     
     
       5. An igniter according to  claim 1 , wherein five to nine, in particular five to seven electrode branches come out of the head section. 
     
     
       6. An igniter according to  claim 1 , wherein the distance of the neighboring ends of the electrode branches is 2 mm to 7 mm and is preferably greater than 3 mm. 
     
     
       7. An igniter according to  claim 1 , wherein the distance of the ends of the electrode branches from the longitudinal axis of the outer conductor is smaller, preferably only slightly smaller than half the core diameter of an outer thread provided on the outer conductor. 
     
     
       8. An igniter according to  claim 1 , wherein the ignition electrode comprises a center electrode running coaxially to the outer conductor and an electrically conducting head section that is a separate part which is mounted on a front end of the center electrode, wherein the electrode branches come out of said head section. 
     
     
       9. An igniter according to  claim 8 , wherein the head section is a disc or has the form of a disc. 
     
     
       10. An igniter according to  claim 8 , wherein the electrode branches are connected to the head section as a single piece. 
     
     
       11. An igniter according to  claim 8 , wherein the head section is welded to the center electrode, preferably to a blunt end of the center electrode. 
     
     
       12. An igniter according to  claim 8 , wherein the head section is riveted to the center electrode. 
     
     
       13. An igniter according to any one of the  claim 8 , wherein the head section is cut out of a sheet metal and bent together with the electrode branches as a single piece. 
     
     
       14. An igniter according to  claim 13 , wherein the thickness and preferably also the width of the electrode branches increases starting from the ends of the electrode branches in direction of the centre of the disc. 
     
     
       15. An igniter according to  claim 1 , wherein the ends of the electrode branches lie on a circle in a common plane and point into directions, which are tilted with the same angle with respect to an axis which is vertical on the common plane and runs through the centre of the circle. 
     
     
       16. An igniter according to  claim 1 , wherein the ends of a first group of two to four electrode branches lie in a first plane and the ends of at least one another group of electrode branches lie in another plane, which runs parallel to the first plane, and in that the ends of the first group of the electrode branches lie on a first circle and the ends of the electrode branches of the at least one another group lie on another circle, and in that the circles are arranged coaxially to one another and that when looking into the direction of the axis running through the centre of the circle, each electrode branch of every single group lies between any two electrode branches of the other groups. 
     
     
       17. An igniter according to  claim 16 , wherein exactly two groups of electrode branches are present. 
     
     
       18. An igniter according to  claim 12 , wherein the ends two, three or four electrode branches lie in a common plane. 
     
     
       19. An igniter according to  claim 15 , wherein the longitudinal axis of the outer conductor goes through the centre(s) of the circle(s). 
     
     
       20. An igniter according to  claim 1 , wherein the electrode branches are embedded in an insulating material. 
     
     
       21. An igniter according to  claim 20 , wherein the electrode branches are completely embedded in the insulating material and end on the surface or tightly below the surface of the insulator. 
     
     
       22. An engine, in particular an engine for a vehicle, with one or several combustion chambers, which each have an igniter having the features according to one of the previous claims.